Apparatus for vapor coating base material in powder form



Feb. 22, .1955 R J, pREsTwQQD ET AL 2,702,523

APPARATUS FOR VAPOR COAVTING BASE MATERIAL IN POWDER FORM Filed June 9.1947 i INVENTORS. Don S. Mcrrz'z; Bni/ze' J Pres'wood the method foundto be feasible.

United States Patent O APPARATUS FOR VAPOR COATING BASE MATERIAL INPOWDER FORM Rene J. Prestwood, St. Louis, Mo., and Don S. Martin,

Ames, Iowa, assignors to the United States of America as represented bythe United States Atomic Energy Commission Application June 9, 1947,Serial No. 753,516

l Claim. (Cl. 118-48) This invention relates to apparatus useful in thepreparation of radioactive products and more particularly to apparatususeful in coating base materials with radioactive materials.

In many operations involving vradioactive materials it is necessary thatthe radioactive material be in intimate contact with some base materialand preferably coated on the surface thereof. For example, in theconstruction of a neutron source, polonium or some other alpha particleemitting substance is coated on the surface of berylliumor some otherbase material which emits neutrons in response to alpha particlebombardment. It has been determined that the eliciency of such a neutronsource canbe increased vby depositing the radioactive material on a basematerial which is in a finely divided state. Similarly, for variousradioactive products and/or for various purposes, 1t is desirable todeposit radioactive substances on finely divided base materials incontradistinction to deposition of continuous at films of suchsubstances.

Heretofore, it has been suggested that' a radioactive coating could bedeposited on base materials by the condensation of vaporized radioactivematerials. Apparatus for accomplishing this has been constructed andSuch apparatus is described, for example, by Rona and SchmidtV inZeitschrift fur Physik, volume 48, pages 784-789, May 2l, 1928, theparticular embodiment comprising an elongated cylindrical quartztube-with a constricted-portion near its midpoint. A suitable piece orother arrangement of radioactive material was disposed in theconstricted portion of the tube. The base material was incorporated in a-cup-shaped cavity in the end of a solid copper cylinder which wasinserted in one e11- largedA section of the quartz tube and waspositioned with the cup-shaped cavity adjacent to the constrictedportion of the quartz tube. A suitable gas was admitted through theopposite enlarged section of the quartz tube, while heat from a Bunsenburner was applied to the constricted portion of the tube. As a resultof the heating and of the reduced atmospheric pressure due to theBernoulli effect,4 a portion of the radioactive material was vaporizedand carried by the gas to the base material where the radioactivematerial condensed as a result of the cooling effected by the large heatcapacity and conductivity of the copper cylinder. The excess gas escapedthrough the space between the copper cylinder and the wall of the quartztube.

material cannot be uniformly coated, since the carrier gas does notpenetrate to any great extent into the cupshaped cavitycausingvdeposition of the radioactive material 'on the iirst encounteredbase material. A further disadvantage of previously employed apparatusis that-no provision is made for easily removing the radioactiveproduct\-Obviously any excessive handling during removal results in aloss of the radioactive material which would not only decrease theetliciency of operation but would also contaminate surrounding apparatusand jeopardize the health of the operator. Furthermore, breakage of theapparatus could occur as a result of a carelessly applied ame or similarlocalized heating, would also result in loss of radioactiveprocess arepredetermined and controlled or regulated;

to provide means for removing the radioactive product from the apparatusquickly and easily; to provide means for increasing the penetration ofthe bulk base material by the radioactive vapors; and to provide meanswhereby substantially all of the radioactive vapor formed within theapparatus is deposited on the base metal powder.

The objects of this invention are accomplished by an apparatusconsisting of an elongated container containing a constricted portionwith predetermined gas tlow characteristics which separates thevaporizing section containing an elongated heater or other means ofvaporizing the radioactive material from the condensing sectioncontaining a condenser with appropriate means for circulating coolingliquids surrounding a removable vessel holding the powders to b'ecoated.

It has been found desirable to construct the'elongated container memberof a transparent material, e. g. quartz or Pyrex g'lass, to permitobservation of the progress of the operation. In some embodiments,however, a suitable metal might be employed for increased durability ofthe apparatus or to act as a shield against harmful radiations from theradioactive material. It is also desirable in particular applications toconstruct the contain'eember of two or more materials, for example,

` the inner walls--offtheW elongated container might be It is apparentthat the ellciency of such apparatus I is low since the rates ofheating, cooling, and gas ow .-are not regulated. Excessive heating, forexample,

vaporizes an excessof radioactive material of which a large portion doesnot condense on the base material but rather is carried to other partsof the apparatus or lost from the system. On the other hand, too littleheat does not vaporize a sutcient quantity of radioactive material andis wasteful of the carrier gas, or causes premature deposition of theradioactive material on the intervening parts of the apparatus ratherthan on the base material. Similarly, excessive gas ow would carry alarge portion of the radioactive material beyond the base material whiletoo little gas ow would cause premature deposition and poor penetrationof the radioactive vapors into the cup-shaped cavity. Similar poorpenetration or loss of radioactive material occursA if the rate ofcooling is too large or too small. Even under the best of conditions, afinely divided base constructed of a material chemically resistant tothe radioactive vapors while the outer wall might be constructed of amaterial resistant to the transmission of radiations from theradioactive material.

The introduction of a jet-like constricted portion between thecondensing zone and the vaporizing zone results in more efficient use ofthe radioactive material. That is, by employing the constricted portionof the container member to control the llow characteristics of thecarrier gas through the container, as well as to focus or direct thestream over the base material in the condensing zone, less of theradioactive material is deposited on the Walls of the container.Furthermore, improved penetration of the arrangement of granular basematerial is accomplished. Since the efficiency of operation of the jetdepends upon'the rate of gas ow, a suitable valve is preferablyincorporated in the gas inlet line.

It has been found preferable in the practice of the invention to heatthe vaporizing portion of the elon- `gated container member by means ofan electrical resistance furnace element which substantially surroundsthe vaporizing zone. The temperaturel of the furnace may readily bevaried merely by changing the electrical input to the resistanceelement. It has also been found desirable to employ means such as atemperature sensitive device, e. g., a thermocouple, in reactiveassociation with the vaporizing zone which may be used in conjunctionwith indicating means or :in conjunction with apparatus for theautomatic control ot the external heating means. Although an electricalresistancefurnace element is well adapted for automatic control, it isnot intended that the invention be limited to this one means for heatingsince there are other heating methods which could be used conveniently.linduced heating of the radioactive material within the vapoiizing zoneby external high frequency alternating electric currents would besuitable and would be amenable to automatic control. Various types offlames suitably equipped with control means might be desirable in someembodiments of the invention.

To facilitate the deposition of the radioactive material on the basemetal powder, it has been found to be advantageous to circulate acoolant through the member in which the base metal powder isincorporated. The type of coolant used depends upon the temperaturerange desired in the condensing zone and although the number of coolantswhich might be conveniently used is almost limitless, water, air, steam,and low melting point alloys have been satisfactorily employed. Sincethe rate of condensation of the radioactive material is partiallydependent upon the rate of flow of coolant, it has been found desirableto incorporate methods of controlling the ow of coolant such as arotometer or a valve in the coolant supply line. In some embodiments ofthe invention it is desirable to employ a temperature sensitive devicelin association with the condensing zone to permit automatic control ofthe rate of coolant iiow.

Another system for regulating the rate of coolant :dow comprises anionization chamber or some other radiation sensitive device locatedadjacent to the cargas exit line. If the rate of cooling decreases to'the extent that a portion of the radioactive vapor is carried beyondthe base material, the ionization chamber detects the Lpresence of theradioactive material in the exit gas and gives a suitable indicationthat the rate of coolant flow should be increased. This can be effectedeither manually or automatically.

The tubular shape of the member in which the base metal powder isincorporated allows the carrier gas to I pass through the volumeoccupied by the bulk base material thereby increasing the penetration ofsaid material by the radioactive vapor and producing a more uniformdeposition. This tubular member is adapted to be easily removed from thecondensing portion of the elongated container member and means isafforded for easily extracting the radioactive product from the tubularmember.

In order to further the understanding of the invention reference is madeto a presently preferred embodiment shown in cross section in Figure lof the drawings made a part of this specification. As shown on thedrawing the apparatus comprises three main sections in a continuousquartz tube 5. The irst main section, 23 is a vaporizing zone whichincludes a source foil 6 consisting of a coating (e. g. electrodepositedcoating) of the radioactive material on a suitable base such as gold orplatinum, and a heating means 7, which is preferably a resistancefurnace element. The second main section 26 is a converging portion ofthe quartz tube defining a jet 8 which increases the velocity andcontrols the ow characteristics of the carrier gas and which enhancespenetration of the bulk base metal powder. The third section 24- is thecondensing zone in which the coating deposition takes place, and whichincludes a chambered condenser unit 9 through which a coolant,preferably steam or water, is circulated. The condenser unit 9 is ahollow container of U-shaped cross section and has a cup-shapedre-entrant portion 10 in which the base metal powder is retained by abored plug 1l, a platinum gauze l2 and a tubular platinum sleeve 13. Thecondenser assembly lits into an enlarged section of the quartz tube sothat the reentrant portion liti is positioned adjacent to the jetElongated coolant ducts 2l and 22 for the ingress and egress of coolantare connected with the chambered condenser unit 9. Chambered condenserunit 9 is of U-shape in diametrical cross section. This condenser unitis supported adjacent an apertured disc-shaped end wall 28 of thirdsection 24. The aperture in the discshaped end wall converges inwardlyfrom the plane of the surface of the end wall which faces inwardly ofposed within the vaporizing zone.

the third section to thereby provide jet 8. The plane of least diameterof the aperture is substantially in the plane of the outer surface ofend wall 28. The walls defining the aperture 29 connect and merge withthe small end of converging second section 26, thereby forming a venturior venturi section or venturi duct.

A gas inlet id and control valve 19 lets gas into the vaporizing zoneand a gas outlet 15 communicates with the bottom of the cup-shapedre-entrant portion l0. An elongated re-entrant well 16 extending intothe vaporizing zone permits insertion of a thermocouple or othertemperature sensitive device which, with suitable auxiliary means, canbe used for the re lation of the heating means '7. A rubber stopper Il?is employed to seal the unit.

In a typical operation to coat beryllium with polomium for thepreparation of a neutron source, with the condenser unit 9 in positionand beryllium metal powder 18 incorporated in -the cup-shaped portion10, the source foil d on which polonium had been previously deposited,by well-known methods, is heated to about 1000 C. to 1l00 C. by means offurnace 7 to eEect vaporization of the radioactive material. Thepolonium melts at about 255 C. lt has a vapor pressure of 1x10-4millimeters of mercury at 290 C. and of 2() millimeters of mercuray at760 C. Heliuinis started into the system through inlet lid before or assoon as the temperature of the source foil t5 rises to 100 C. in orderto minimize the radioactive material which is deposited within theapparatus. The helium ow is adjusted for optimum operating leiciencywhen the source foil ti is heated to operating temperature. The gascarries the vaporized polonium through the jet 8 and through the metalpowder 1d, which is kept cool by steam or water owing through thecondenser unit 9. The vaporized poloniurn condenses on the berylliumparticles, the gas passing out of the system through outlet i5. Thisoutlet gas is routed through a suitable trap, not shown, which retainsany radioactive material which is not deposited. After the tiow hascontinued long enough to obtain the desired coating, thefurnace 7 isturned o, but the helium ow is continuetlat a reduced rate until tenminutes after the source foil has cooled down below C. When the coatedpowder is cool it is removed from the condenser by removing the stopper117 and withdrawing the condenser unit 9 from the quartz tube. Asuitable source container may then be slipped over the platinum sleeve13, the unit inverted, and the coated powder pushed out by means of arod inserted into outlet tube l5.

A rate of helium flow of about 50 milliliters per minute at a pressureof 600 millimeters of mercury and at room temperature with a jet ti of 3millimeters diameter will satisfactorily prepare in about two hours apoloniumberyllium neutron source comprising sumcient beryllium powder,which passes through (-0 mesh and is held on 100 mesh screen, to ll acylindrical container 1/2 inch in diameter and l/ inch deep.

Although two hours were taken in the example, the time needed fordepositing the radioactive material depends on many variables such asthe characteristics of the apparatus resulting from its design, and theconditions used in operating it. The size of 'et 8 is one of the moreimportant design variables. e rate of ow of the helium and thetemperature of the source foil 6 are important operating variables. Thetemperature of the source foil 6 can be increased above 1l00 C. and thiswill cause an acceleration in the rate of radioactive materialdeposited. However this will also cause a reduction in the etliciency ofthe operation.

Although specific apparatus and a specilic operation have been describedit is not intended that the invention be limited thereby. The elongatedcontainer member could be constructed of other materials and with othercross-sectional shapes and. might conveniently be constructed in two ormore separate sections which could be fastened together.` .Such aconstruction would be particularly useful in an embodiment of theinvention in which it was desired to substitute various sized jets toachieve specific deposition effects. The radioactive material could besupported within the Vaporizing zone by other means than that described,for example, it could be incorporated in a crucible which is dis- Manyother modifcations of the apparatuscould be made without affecting thespirit or scope of this invention.

What is claimed is:

Apparatus for coating base material in powder form by vapor depositioncomprising an elongated receptacle having a first section, a secondsection and a third section, said receptacle first section being open atiirst and second ends and being of substantially uniform cross sectionand beingv adapted to support vaporizable material therein, heatingmeans supported in surrounding relation to said receptacle rst section,a gas duct connected to the first end of the receptacle first section,the receptacle second section being of venturi shape with receptaclethird section with the cup-shaped re-entrant the long portion of theventuri connected to the second y end of the first section, the secondend of the venturi being a disc-like portion the plane of which isnormal to the axis of the receptacle venturi section and which includesa centrally located aperture aring inthe direction outward of theventuri section, the receptacle third section being of substantiallyuniform cross section and being connected at one end to the outerportion of the disc of` the receptacle second section, a condenserhaving an outer conguration similar to the portion facing the venturi, abored plug and gauze screen supported in the bottom of the cup-shapedre- .entrant portion thereby providing in the remainder of thecup-shaped re-entrant portion a receptacle for the base material.

References Cited in the tile of this patent UNITED STATES PATENTS1,881,616 Ives Oct. 11, 1932 1,954,995 Harrison Apr. 17, 1934 1,994,668Russell Mar. 19, 1935 2,157,478 Burkhardt et al. May 9, 1939 2,164,332Macksoud July 4, 1939 2,258,374 Amati Oct. 7, 1941 2,416,211 Osterberget a1. Feb. 18, 1947 OTHER REFERENCES Ser. No. 233,445, Berghaus et al.(A. P. C.), published May 4, 1943.

